1. Field of the Invention
The present invention relates to an ultrasonic horn used in a bonding apparatus and more particularly to an ultrasonic horn that transmits ultrasonic vibrations to a capillary with a bonding wire passing therethrough.
2. Prior Art
In ultrasonic horns used in, for instance, a wire bonding apparatus, a capillary is fastened in a capillary attachment hole that is formed in the tip end portion of the horn main body. Conventional capillary fastening structures can be categorized into two types.
In the first type, a slit is formed vertically along the axial center of the horn main body in a portion of the capillary attachment hole, and the capillary is fastened in place by tightening the area surrounding this slit with a bolt. This type of fastening structure is disclosed in, for instance, Japanese Patent Application Laid-Open (Kokai) Nos. H6-163648, H6-283578 and H11-121546. In particular, in Japanese Patent Application Laid-Open (Kokai) No. H6-163648, the ultrasonic horn is formed with a slit, and it is further provided with a stress compensation hole that is larger than the capillary mounting hole. In Japanese Patent Application Laid-Open (Kokai) No H6-283578, a trench is formed in the ultrasonic horn so that it works in a bifurcated fashion. In Japanese Patent Application Laid-Open (Kokai) No H11-121546, a capillary-holding hole is formed at one end of the ultrasonic horn, and a vertical slit is provided from this hole in the opposite side t the tip end of the horn.
In the second type, a slit is formed so as to extend through the capillary attachment hole from the tip end of the horn main body, the area of the capillary attachment hole is endowed with an elastic force, and a screw hole is formed on one side of the slit. A bolt is screwed into the screw hole and tightened. When the bolt is thus tightened, the other side of the slit is pressed so that the slit and capillary attachment hole are widened. After the capillary is inserted into the capillary attachment hole, the bolt is loosened and removed from the horn main body. As a result, the capillary is fastened in place only by the elastic force of the horn main body. This fastening structure is described in Japanese Utility Model Application Laid-Open (Kokai) No. H5-59840.
However, the above capillary fastening structures have problems.
The first type fastening structure has the following problems:
(1) Since the structure involves a bolt that is used to fasten the capillary to the horn main body, the masses on the left and right sides of the horn main body with respect to the axial center of the horn main body are different. As a result, the ultrasonic vibration at the tip end portion is disturbed, and the accurate transmission of ultrasonic waves to the capillary is hindered.
(2) The bolt and the screw hole in the horn main body wear with time. As a result, changes in the configuration of the ultrasonic vibration occur, causing the capillary tightening force to be unstable.
(3) The tightening torque of the bolt must be sufficiently strong so that the bolt itself does not show any relative movement or slippage with respect to the horn main body. Such a relative movement or slippage is derived, for instance, from the bolt becoming loose during the bonding operation and from wear in the bolt. The limit of the tightening torque of the bolt is determined by the size and strength of the head of the bolt that is used for tightening. Currently, the diameter of the head of the bolt generally used in such cases is approximately 3 mm, and this bolt head can withstand a torque of approximately 2 kg. However, in ultrasonic vibration at 80 kHz, the weight of the bolt itself is too heavy. Thus, with the tightening torque of approximately 2 kg, the tightening force is overcome by the force arising from the acceleration of the ultrasonic vibration, thus causing the bolt to slip on the horn main body, and further causing wear of the bolt, energy loss and unstable vibration. Furthermore, with the tightening torque of approximately 2 kg, the shock generated by gravitational acceleration during movement of the capillary cannot be suppressed when ultrasonic oscillation is at 100 kHz, thus causing unstable vibrations in the capillary.
(4) The bolt is generally tightened using a torque driver in order to ensure a constant tightening force. However, torque drivers are expensive, and also they have drawbacks of showing changes in tightening torque unless periodically adjusted. As a result, troubles such as faulty tightening, etc. can occur easily.
The second type fastening structure has the following problems:
(1) Since a screw hole is formed on one side of the slit, the masses on the left and right sides of the horn main body with respect to the axial center of the horn main body are different as in the above first type. Thus, the ultrasonic vibration at the tip end portion is disturbed, hindering the accurate transmission of ultrasonic waves to the capillary.
(2) In order to ensure a sufficient grip in the tip end portion of the horn main body that is open, it is necessary to increase the thickness of the tip end portion so as to increase the elastic force of the horn main body. However, in the structure for gripping the capillary from the left and right sides, the capillary may be damaged before a sufficient gripping force is applied to the capillary.
(3) Furthermore, so as to transmit ultrasonic vibration efficiently from the vibrator provided in the rear portion of the horn main body, it is generally necessary to design the tip end portion of the horn main body slender. There are restrictions also on the diameter and length dimensions of the horn main body in view of performance. Currently, the thickness of the capillary gripping portion located in the tip end portion of an ordinary horn main body is approximately 1 mm. The elastic force obtained from such a thickness is easily overcome by the shock generated by gravitational acceleration during movement of the capillary in ultrasonic oscillation at 60 kHz or greater; and as a result, the capillary is not retained.
(4) Since the tip end portion of the horn main body is opened by a slit, such a tip end portion is caused to undergo repeated slight expansions and contractions by the ultrasonic vibration. As a result, tensile and compressive stresses are generated inside the capillary, causing cracks, failing to generate a sufficient holding force. Furthermore, destructive accidents can occur as a result of changes in strength, etc. even in ordinary use.
Accordingly, the object of the present invention is to provide an ultrasonic horn for a bonding apparatus that suppresses the generation of unnecessary vibrations other than the ultrasonic vibration at the fundamental frequency, improves the bondability, and realizes stable gripping for the capillary by a simple structure.
The above-object is accomplished by a unique structure for an ultrasonic horn used in a bonding apparatus in which a capillary is fastened in a capillary attachment hole formed in the tip end portion of the horn main body, and in the present invention:
the capillary attachment hole is formed smaller than the capillary in diameter,
a jig insertion hole is formed in the horn main body so as to communicate with the capillary attachment hole, and
the area surrounding the capillary attachment hole and jig insertion hole is formed as a closed structure, and wherein
the jig insertion hole is pushed open using a jig, thus widening the capillary attachment hole so that the capillary attachment hole becomes larger than the capillary and the capillary is inserted into the capillary attachment hole, and
the capillary is fastened in place by the elastic force generated when the capillary attachment hole returns to its original shape, without requiring any component other than the horn main body for holding the capillary.
In the above structure, the horn main body is formed so that it is symmetrical with respect to its own axial center.
Furthermore, the jig insertion hole communicates with the capillary attachment hole either directly or via a slit.